In a vehicle such as an automobile, generally, a seat having a seat portion for seating an occupant and a back portion located behind the occupant is provided with a seat belt apparatus for restraining the occupant. Such a seat belt apparatus has a webbing for restraining the occupant, a seat belt retractor for retracting the webbing, a guide anchor provided on the vehicle body side to guide the webbing, a belt anchor for fixing the webbing to the vehicle body side, a buckle disposed on the side of the seat, and a tongue disposed on the webbing, and restrains the occupant by inserting the tongue into the buckle. In such a seat belt apparatus, one end of the webbing is fixed to the belt anchor, and the other end of the webbing is passed through the guide anchor and connected to the seat belt retractor.
In general, the seat belt retractor has a spool for retracting the webbing to restrain the occupant, and the spool is always urged in the retracting direction by a power spring. When the elastic force of such a power spring is large, there are some problems such as a large resistance during withdrawal of the webbing and a discomfort feeling during wearing the seat belt. On the other hand, when the elastic force of the power spring is small, the withdrawn webbing is difficult to retract. Now, a seat belt retractor provided with a motor to assist the retracting of the webbing has already been proposed (see, for example, Japanese Unexamined Patent Application Publication Nos. 2005-53422 and 2008-280010). In such a motorized seat belt retractor, when the distance between vehicles becomes smaller than a predetermined distance, the motor is sometimes controlled so that it repeatedly pulls the webbing in order to send a signal to the occupant.
As described in Japanese Unexamined Patent Application Publication Nos. 2005-53422 and 2008-280010, a motorized seat belt retractor is generally provided with a clutch for switching between a state where the drive torque of the motor is transmitted to the spool and a state where the drive torque of the motor is not transmitted to the spool. In the invention described in Japanese Unexamined Patent Application Publication No. 2008-280010, there is provided an operating noise reduction means for preventing the emanation of abutment noise between a pawl (clutch member) and a stopper member upon switching of the clutch.